1. Field of the Invention
The present disclosure generally relates to the field of fabricating semiconductor devices, and, more particularly, to process control and monitoring techniques in lithography processes.
2. Description of the Related Art
Today's global market forces manufacturers of mass products to offer high quality products at a low price. It is thus important to improve yield and process efficiency to minimize production costs. This holds especially true in the field of semiconductor fabrication, since here it is essential to combine cutting edge technology with volume production techniques. It is, therefore, the goal of semiconductor manufacturers to reduce the consumption of raw materials and consumables, while at the same time improve product quality and process tool utilization. For example, in manufacturing modern integrated circuits, several hundred individual processes may be necessary to complete the integrated circuit, wherein failure in a single process step may result in a loss of the complete integrated circuit. This problem is even exacerbated in current developments striving to increase the size of substrates, on which a moderately high number of such integrated circuits are commonly processed, so that failure in a single process step may possibly entail the loss of a large number of products.
Therefore, the various manufacturing stages have to be thoroughly monitored to avoid undue waste of man power, tool operation time and raw materials. Ideally, the effect of each individual process step on each substrate would be detected by measurement and the substrate under consideration would be released for further processing only if the required specifications, which would desirably have well-understood correlations to the final product quality, were met. A corresponding process control, however, is not practical, since measuring the effects of certain processes may require relatively long measurement times, frequently ex situ, or may even necessitate the destruction of the sample. Moreover, immense effort, in terms of time and equipment, would have to be made on the metrology side to provide the required measurement results. Additionally, utilization of the process tool would be minimized since the tool would be released only after the provision of the measurement result and its assessment. Furthermore, many of the complex mutual dependencies of the various processes are typically not known, so that an a priori determination of respective optimum process specifications may be difficult.
The introduction of statistical methods, also referred to statistical process control (SPC), for adjusting process parameters significantly relaxes the above problem and allows a moderate utilization of the process tools, while attaining a relatively high product yield. Statistical process control is based on the monitoring of the process output to thereby identify an out-of-control situation, wherein a causality relationship may be established to an external disturbance. After occurrence of an out-of-control situation, operator interaction is usually required to manipulate a process parameter so as to return to an in-control situation, wherein the causality relationship may be helpful in selecting an appropriate control action. Nevertheless, in total, a large number of dummy substrates or pilot substrates may be necessary to adjust process parameters of respective process tools, wherein tolerable parameter drifts during the process have to be taken into consideration when designing a process sequence, since such parameter drifts may remain undetected over a long time period or may not be efficiently compensated for by SPC techniques.
Recently, a process control strategy has been introduced and is continuously being improved, allowing enhanced efficiency of process control, desirably on a run-to-run basis, while requiring only a moderate amount of a measurement data. In this control strategy, the so-called advanced process control (APC), a model of a process or of a group of interrelated processes, is established and implemented in an appropriately configured process controller. The process controller also receives information including pre-process measurement data and/or post-process measurement data as well as information related, for instance, to the substrate history, such as type of process or processes, the product type, the process tool or process tools in which the products are to be processed or have been processed in previous steps, the process recipe to be used, i.e., a set of required sub-steps for the process or processes under consideration, wherein possibly fixed process parameters and variable process parameters are included, and the like. From this information and the process model, the process controller determines a controller state or process state that describes the effect of the process or processes under consideration on the specific product, thereby permitting the establishment of an appropriate parameter setting of the variable parameters of the specified process recipe to be performed with the substrate under consideration in order to keep the process result close to the preset target.
One important process for the fabrication of microstructure devices such as integrated circuits and the like is the transfer of a required pattern from a template or mask to the actual substrate. That is, the fabrication of microstructures requires tiny regions of precisely controlled size to be formed in a material layer of an appropriate substrate, such as a silicon substrate, a silicon-on-insulator (SOI) substrate or any other suitable carrier material. These tiny regions of precisely controlled size are generated by patterning one or more material layers provided on the substrate by performing lithography, etch, implantation, deposition, oxidation processes and the like, wherein typically, at least in a certain stage of the patterning process, a mask layer is to be formed over the one or more material layers to be treated in order to define these tiny regions. Generally, a mask layer may consist of or may be formed by means of a layer of photoresist that is patterned by a lithography process, which frequently is implemented in the form of an optical or photolithography process. During the photolithography process, the resist may be spin coated onto the substrate surface and is then selectively exposed to ultraviolet radiation through a corresponding lithography mask, such as a reticle, thereby imaging the reticle pattern into the resist layer to form a latent image therein. After developing the photoresist, depending on the type of resist, i.e., positive resist or negative resist, the exposed portions or the non-exposed portions are removed to form the required pattern in the layer of photoresist material. Based on this resist pattern, actual device patterns are then formed by further manufacturing processes, as specified above. The size and shape of the finally obtained features thus strongly depends on the quality of the mask formed on the basis of the resist material and thus makes the lithography process a very important process step in order to precisely define the shape and size of any circuit components, wherein a high degree of precision is also required for appropriately aligning the various mask layers that have to be provided during the entire manufacturing process for completing a complex integrated circuit. That is, typically, a plurality of mask layers or levels are required which are established on the basis of dedicated templates or lithography masks in order to appropriately complete the complex circuit elements, wherein any deviations in size and shape as well as any undue misalignments may generally contribute to significant device variabilities or even total failures upon completing the manufacturing process. For example, a plurality of lithography steps are typically required for providing the various semiconductor-based circuit elements, such as transistors and the like, for instance for forming sophisticated gate electrode structures, implementing appropriate dopant profiles in the semiconductor materials and the like, wherein critical dimensions of 50 nm and even less may have to be realized on the basis of sophisticated lithography processes. In subsequent levels of the semiconductor device, further sophisticated lithography processes may be required, for instance, for defining contact elements for connecting to the semiconductor-based circuit elements, for forming sophisticated metallization systems, which also typically comprise a plurality of stacked metallization layers, and the like.
Since lithography processes are typically extremely cost-intensive process steps due to the complex lithography tools required, great efforts are being made in precisely monitoring and controlling the lithography process module. For example, a typical lithography process may comprise a plurality of pre-exposure processes, such as the deposition of an appropriate resist material, a pre-treatment of the resist material, for instance in the form of heat treatments, and the like. Thereafter, the actual exposure process is performed, wherein, among other things, the amount of energy deposited within the resist material may significantly affect the size of the corresponding resist features after developing the exposed resist material. Furthermore, typically, one or more post-exposure processes in the form of heat treatments and the like may be required in addition to the actual development process. Hence, powerful process control strategies, for example on the basis of APC and SPC, have been implemented by the overall manufacturing process in order to provide superior process quality. That is, the process control and monitoring techniques strive to maintain the process result, i.e., the size and shape of the developed resist features, as closely as possible at a desired target value. Thus, the APC controllers may typically comprise an appropriate model which operates on the basis of a reasonable amount of measurement data that indicates critical dimensions of previously processed substrates, in order to keep the process output at a desired target level that is given by design rules for a specific device layer. To this end, the APC system may provide appropriate parameter values for at least one parameter that may have a significant influence on the final process output. As explained above, the energy or exposure dose used during the exposure process represents a convenient process parameter of the exposure process, which may be appropriately manipulated in order to re-adjust the finally obtained critical dimensions of the resist features. For example, when the measurement data indicate a deviation of the measured resist features from the target value, for instance when the measured resist features are greater compared to the target value, the APC system may determine an appropriate target value for the parameter exposure dose that is to be used in the subsequent exposure process in order to bring back the resulting resist features to the target value, which, in this example, would require an increase of exposure dose. Thus, on the basis of previous measurement data and a predictive model, the APC system may provide re-adjusted exposure dose, which is expected to produce process results closer to the target value.
In complex production facilities for fabricating complex semiconductor devices, typically, a plurality of process tools have to be provided in view of throughput considerations, wherein, to the effect that typically the number of different products is to be processed at the same time within the facility, the different process tools may be used for different processes and products, depending on the overall scheduling regime in the facility. Consequently, a huge amount of measurement data and process parameters may have to be processed by the APC system in order to produce acceptable process results for any product type and any device layer thereof for any combination of process tools used, for instance for performing the lithography process flow. Similarly, in the statistical process control, the tool data and measurement data have to be monitored in order to identify an out-of-control situation. For example, although very efficient APC strategies may be applied, nevertheless out-of-control situations may occur which may remain undetected by the APC system, while nevertheless requiring any corrective activities, since generally a significant shift of the overall process parameters may occur, which may result in a significantly yield loss if the out-of-control situation remains undetected for a pronounced production time. For example, a shift in a metrology tool, which may provide measurement data to the APC system, may cause the APC system to provide re-adjusted exposure dose values to the various lithography tools, which are used for performing the specific exposure process under consideration. In this case, a more or less pronounced deviation in the parameter values for adjusting the parameter exposure dose may be observed, which may, however, be within acceptable ranges, while at the same time the resulting process output may significantly drift away from the target critical dimension of the resist features. In this situation, the APC system may appropriately work, while at the same time the inferior process results may remain non-detected and may enter further subsequent processes, such as etch processes, after which a reworking of the substrates may no longer be possible. To this end, efficient SPC strategies have to be provided which, however, may require the tracking and analyzing, as well as the selection of appropriately set process limits, for a very large number of individual processes, thereby significantly contributing to additional effort and also to a pronounced probability of creating false alarms, which may also reduce overall throughput and thus profitability.
The present disclosure is directed to various methods and systems that may avoid, or at least reduce, the effects of one or more of the problems identified above.